Research On Refractive Index Sensor Based On Optical Fiber S-cone Microstructure

Refractive index(RI)is an important physical parameter that reflects the optical properties and constituent concentrations of liquids,therefore,its accurate measurement is of great significance.Fiber-optic RI sensor has the advantages of small size,fast response,high sensitivity and anti-electromagnetic interference,which can be widely used in food processing,biochemical pharmacy,environmental monitoring and other fields.With the development of micro-processing technology in optical fiber,the compact fiber micro-structure RI sensor has gradually become a research focus.In this paper,the S fiber taper(SFT)micro-structures are used as the RI sensing elements.According to the structure and spectral characteristics of SFT,the RI sensors with different features are proposed and demonstrated by combining with the related optical fiber sensing technologies.The concrete research contents are as follows:1.A reflective intensity-demodulated SFT RI sensor based on edge filtering method is proposed and researched.The reflective intensity-demodulated RI sensing structure is constructed by cascading the SFT with a fiber Bragg grating(FBG).By measuring the peak power of the FBG reflection,the temperature-insensitive RI measurement with a high sensitivity of 366.69 dB/RIU can be achieved during the RI range of 1.3330 RIU-1.3988 RIU.In order to improve the quality of the sensing spectra and compact the size of the sensing element,an SFT RI sensor based on fiber ring laser cavity is proposed and researched.The temperature-compensated RI measurement with a sensitivity of 269.76 dB/RIU can be achieved during the RI range of 1.3330 RIU-1.4060 RIU.2.A dual-point transmissive wavelength-demodulated SFT RI sensor and a multi-channel reflective intensity-demodulated SFT RI sensor based on wavelength-division multiplexing are proposed and researched.The transmissive sensor takes the advantage of the superposition of the interference spectra,and the dual-point RI measurement without crosstalk can be achieved by measuring the center wavelengths of the interference notches formed by the cascaded SFTs.The re:flective sensor takes the advantage of the multi-port optical coupler and the tree topology structure.By measuring the peak power of the reflection of each channel,the independent multi-channel RI measurement can be achieved.3.An SFT RI sensing system based on fiber loop ring-down spectroscopy is proposed and researched.The SFT is used as a loss control device to construct the compensated ring-down cavity,and the configuration parameters of the cavity are analyzed and optimized.By measuring the ring down time of the ring down pulse signal,the RI measurement with a high sensitivity of-3128.954 ?s/RIU can be achieved during the RI range of 1.3330 RIU-1.3682 RIU.In addition,according to the characteristics of ring-down spectroscopy,a RI measurement software based on Lab VIEW is designed to improve the automation of the system.